Heat transfer occurs in many areas of the air conditioning and refrigeration engineering and in the process and energy engineering. Shell and tube heat exchangers are often utilized in these areas for heat transfer. A liquid flows hereby on the inside of the tube in many applications, which liquid is cooled off or heated depending on the direction of the heat flow. The heat is given to the medium on the outside of the tube or is removed from said medium. Tubes, which are structured on both sides, constituting the state of the art, are utilized in shell and tube heat exchangers instead of plain tubes. This intensifies the heat transfer on the inside of the tube and on the outside of the tube. The heat-flux is increased, and the heat exchanger can be built more compactly. As an alternative, it is possible to maintain the heat-flux and to lower the driving temperature difference, thus enabling a more energy-efficient heat transfer.
Structured heat-exchanger tubes for shell and tube heat-exchangers have usually at least one structured area and plain ends and possibly plain center lands. The plain ends and plain center lands define the structured areas. In order for the tube to be able to be installed without any problems into the shell and tube heat-exchanger, the outer diameter of the structured areas may not be larger than the outer diameter of the plain ends and plain center lands.
Integrally rolled finned tubes are often being utilized as structured heat-exchanger tubes. Integrally rolled finned tubes are finned tubes where the fins are formed out of the wall material of a plain tube. Finned tubes have on their outside annularly or helically extending fins. They have in many cases on the inside of the tube a plurality of axially parallel or helically extending fins, which improve the heat-transfer coefficient on the inside of the tube. These inner fins extend with a constant cross-section parallel to the axis of the tube or in the form of helixes at a specific angle to the axis of the tube. The higher the inside fins, the greater is the improvement of the heat-transfer coefficient. The manufacture of such tubes is described, for example, in DE 23 03 172. It is of importance hereby that by using a profiled mandrel to produce the inner fins, which use is disclosed in said patent, the dimensions of the inner and the outer structure of the finned tubes can be adjusted independently of one another. Thus both structures can be adapted to the respective requirements and thus the tube can be designed at an optimum.
Lately many possibilities have been developed to further increase, depending on the use, the heat transfer on the outside of integrally rolled finned tubes by providing the fins on the outside of the tube with further structural characteristics. For example, in the case of condensation of refrigerants on the outside of the tube, the heat-transfer coefficient is clearly increased when the fin flanks are provided with additional convex edges (U.S. Pat. No. 5,775,411). It has proven to increase performance during boiling of refrigerants on the outside of the tube when the channels between the fins are partly closed so that cavities open to the outside through pores or slots are created. These essentially closed channels are in particular created by bending or tilting the fin (U.S. Pat. Nos. 3,696,861, 5,054,548), by splitting and flattening the fin (DE 27 58 526, U.S. Pat. No. 4,577,381), and by grooving and flattening the fin (U.S. Pat. No. 4,660,630, EP 0 713 072, U.S. Pat. No. 4,216,826).
The mentioned improvements in performance on the outside of the tube have the result that the main share of the entire heat-transfer resistance is shifted to the inside of the tube. This effect occurs in particular during small flow velocities on the inside of the tube; thus, for example, during a partial-load operation. In order to significantly reduce the entire heat-transfer resistance, it is thus necessary to further increase the heat-transfer coefficient on the inside of the tube. This would principally be possible through an increase of the height of the inner fins which, however, is technically difficult to do because of the increasing, strong deformation of the material, and furthermore results in a heavy weight of the structured tube. A heavy weight is, however, undesired for cost reasons.